Interconnection networks are widely used in telephone exchanges. The well-known crossbar exchange with its separate connections available between every source and every destination is however expensive. An alternative type of interconnection network has been proposed which consists of a number of switches each having a plurality of source ports and destination ports with the ability to switch any of its sources to any of its destinations. Several stages of rows of such switches are provided with the destination ports of one stage being connected to the source ports of the next stage. Each switch can have quite a small number of source and destination ports, for example only two or four each, however, it is possible by appropriately connecting the stages to provide a large fully interconnected network.
In general, for switches having b.times.b source and destination ports a total of nb.sup.n-1 switches can be arranged in n stages of b.sup.n-1 switches in each stage to give a b.sup.n .times.b.sup.n network.
A network may carry information either in serial form or in parallel using multiple connections. A network may be switched either by using control paths which are separate from the information paths or else by sharing the same paths both for control and for information. Networks which carry information in parallel form or which have separate control paths are complex and require a large number of interconnections between network components. Networks which carry information serially and which provide switching along the same paths as the information signals are termed serial switched networks and are inherently simpler and require considerably fewer circuit interconnections.